One of the most common forms of email is provided by Email Service Providers (ESPs) such as Yahoo! Mail, Microsoft Hotmail, Google GMail, and other web-based email services.
Generally, such ESPs direct users running web-browsers to a cluster of computers which provide an email application to the user via the web-browser interface. However, other methods of accessing free email services, such as Post Office Protocol (POP) and Internet Message Access Protocol (IMAP) may be utilized.
ESPs use a multitude of processing devices to implement their services. Generally, the devices have dedicated sets of functionality within the service provider's system. Some devices serve as storage devices, user data stores and message transfer devices. Messages directed to users having accounts at an ESP are directed to the ESP's Message Transfer Agents (MTAs), which work with other devices within the ESP's server structure. In an architecture having a large number of mailboxes in a single location, mail acceptance servers are typically separated from storage servers, and there are generally many machines of each type.